1. Field of the Intention
This invention relates to a roller member for conveying sheets by frictional force thereof in contact with the sheets and, more particularly, to a roller member capable of accurately conveying and separating the sheets even for the sheets in any size, and a sheet conveyance apparatus as well as an image forming apparatus such as a photocopier, a printer, a word processor, a facsimile machine, using this member.
2. Related Background Art
An example of a laser beam printer equipped with a conventional sheet feeding apparatus is shown in FIG. 10. This printer separately feeds one by one the sheets P contained in a sheet cassette 101 in a stacking manner by a feeding roller 102a as a sheet feeding apparatus and a roller 102b rotating integral with the feeding roller 102a, and a separation pad 108 capable of contacting to the feeding roller 102a or the roller 102b. The sheet is conveyed to the image forming section 106 including, e.g., a photosensitive drum 106a and a developing unit 106b by conveyance rollers 104a, 104b and registration rollers 105a, 105b, and the toner image formed at the image forming section 106 is transferred to be formed as image. The sheet P after the toner image is transferred is conveyed to a fixing unit 107 to fix thermally the toner image, and is delivered to A delivery portion 108.
The separation pad 103 is structured of an elastic member such as a rubber or the like in contact with the feeding roller 102a, and a surface 106 thereof is furnished with polishing processing or slicing processing in a direction perpendicular to the sheet feeding direction, thereby separating the recording sheet P with frictional characteristics.
The conventional sheet feeding apparatus described above however, raises the following problems. For example, where feeding force is set largely in aiming at a thick sheet of about 200 g/m2, if a thin sheet of about 60 g/m2 is tried to be fed, the sheets in a plural number are inevitably fed at the same time. To separate the sheets, some plans, are used such that pushing force exerted to the feeding roller 102a of the separation pad 103 is largely set to raise nipping pressure or that the separation pad 103 is made in use of a material having a higher frictional coefficient.
Even where the pushing force of the separation pad 103 is increased as described above, no effect is given except the nipping portion, so that considerably large pushing force is needed to operate separation function only by the nipping portion. Large drive force is therefore required for the feeding roller 102a, so that the drive motor is needed to have a large capacity, and so that this may raise a problem that extraordinary noises called as “screaming” tend to occur where the frictional force among the separation pad 103, the feeding roller 102a, and the roller 102b is strong.
In a case that a material having a high frictional coefficient is used for the separation pad 103, the fictional coefficient may be changed from wearing due to durable situation or the like, and therefore, this raises a problem that the feeding property may be dropped.
In addition, in a case of the separation pad 103 having a high frictional coefficient, a thin sheet fed from the feeding roller 102a way hit the separation pad 103 during conveyance of the thin sheet, thereby raising a problem that the sheet front end receives damages, or namely, “front edge folding” may occur.
Substantially the same phenomenon may occur in sheets being left under a high humidity circumstance and absorbing moisture during a sheet conveyance period for the sheets other than the thin sheets.
Moreover, there raises a problem that the separation performance may be dropped due to reduction of the frictional coefficient from attachments of paper powder.